Mold for use in the manufacture of dry rectifiers



Feb. Z, 1954 A. JENKINS 2,667,722

MOLD FOR-USE IN THE MANUFACTURE 0F DRY RECTIFIERS Filed July 16, 1951ATTORNEY INVENTOR Patented Feb. 2, 1954 MOLD FOR USE IN THE MANUFACTUREOF DRY RECTIFIERS Alexander Jenkins, Kings Cross, London, England,assignor to Westinghouse Brake and Signal Company Limited, London,England Application July 16, 1951, Serial No. 236,972

Claims priority, application Great Britain September 18, 1950 2 Claims.

This invention relatesv to the manufacture of dry rectiers of the kindcomprising a body of semi-conducting material carried by a metal supportor holder and is particularly applicable to the manufacture of smallrectifier elements of this kind of such a size, for example, as are usedin alternating current measuring instruments.

Such small rectier elements are troublesome to handle, this disadvantagebeing particularly apparent in the manufacture of so-called crystalrectifiers, such as germanium or silicon rectiers, when securing thesemi-conductor to the metal support or holder.

The manufacture of these crystal rectiers usually involves the step ofcutting wafers of the desired size from a larger rod or ingot of thesemi-conductor material, which is an operation of some intricacy due tothe small size of the wafers.

It is the object of the invention to provide a mould for the manufactureof dry rectiers whereby the moulded semi-conductor is more easilyapplied and secured to the metal support or holder and which also, inthe case of crystal rectiers, eliminates the necessity of cutting wafersfrom rods or ingots of the semi-conductor material.

According to the invention, a mould for the manufacture of dry rectiersof the above kind includes means for casting a body of semi-conductormaterial in the form of a number of projections each of a suitable sizeand shape to t its metal support or holder and connected by a thin web.

The body is divided up into individual elements by cutting or fracturingthe web between the projections, preferably after securing to thesupports or holders.

Certain semi-conductor materials, for example, germanium, when in themolten state, have a high surface tension which makes it impossible tocast elements of the small size contemplated by the present invention bythe normal method of straight forward pouring of the molten materialinto a suitable mould and this difficulty is overcome according to theinvention by subjecting the semi-conductor material and the mould to avacuum, melting the semi-conductor material, and then forcing the moltenmaterial into the mould by destroying the vacuum.

The invention will now be described with reference to the accompanyingdrawing which illustrates one form of mould suitable for carrying outone of the steps of the process, of which Figure 1 is an end view;

Figure 2 is a cross-sectional view taken along the line II-II of Figurel;

y Figure 3 is a view of the other end of the mould;

Figure 4 is a cross-sectional View taken along the line IV-IV of Figure2;

Figure 5 is a perspective view of the two parts of the mould with thecentre plug removed; and

Figure 6 illustrates the cast body when removed from the mould.

According to one method of carrying out the invention, the body ofsemi-conductor material is cast in a mould, comprising a block I ofcarbon or other suitable material such as silica, provided with atapered bore 2 of circular cross section into which ts a similarlytapered plug 3 of such a length that, when it is fully inserted into thebore, it does not extend the full length of the bore so that the end ofthe plug forms the base of a receptacle or crucible 4 of which theportion of the bore extending beyond the end of the plug forms thesides. Radially spaced around the circumference of the plug are providedtwo longitudinally extending flats 5 which, when the plug is insertedinto the bore, form shallow chambers 6 into which the molten materialflows from the crucible as described hereinafter. Radially along thelength of each of the flats recesses 'l are formed. These recesses areconical or of such other shape and size as corresponds with the shapeand size of the metal holders which are to receive the semi-conductor inthe finished rectifier element. 'I'he end of the plug forming the baseof the crucible is provided with a conical recess 8, co-axial with theplug, communicating at its apex through a bore 9 and passages l with thechambers 6.

The tapered plug 3 is inserted into the bore and the mould is supportedso that the two are substantially vertical with the crucible 4 formed atthe end of the bore uppermost. A quantity of the semi-conductor materialis inserted in the crucible and the atmosphere evacuated to a fewmillimetres of mercury. The mould is then heated, by high frequencyinduction for example, until the semi-conductor is molten, whereupon thevacuum is suddenly destroyed. This results in the forcing of the moltenmaterial from the crucible 4 through the passage I0 into the chambers 6and recesses 1. When using some semiconductor materials it is necessaryto take precautions agalnst oxidisation or other chemical changes and/orto prevent the absorption, whilst in its molten state, of certainimpurities. In such cases the vacuum should be destroyed by theintroduction of a suitable gas. For example, when germanium is thesemi-conductor an inert gas such as argon, hydrogen or nitrogen may beused. On the otherhand, theicharacteristics of some semi-conductors areimproved by the addition of certain impurities and this may beaccomplished by the use of a suitable gas to destroy the vacuum.Similarly, although in the example described herein the mould isstatedtobeofY carbon or silica, it will be understood by thoseskiiled in the artthat the choice of material will be governed by a consideration ofAtheeffect that certain materials have upon the semi-conductor materialbeing used.

After cooling, which may be controlled, if desired, to produce afavourable crystalline structure of the material, the plug is loosenedby tapping the propjecting end Il and is withdrawn from the block andthe semi-conductor is removed in the form of a number of projections l2,each-of a suitable size and shape to lit their metal. holders,connected-by.aethin-web I3, somewhat in the form of a comb (see-Fig. 6),

The semi-conductor is then supported inia jig which maybe in the-form ofa perforated sheet while a metal coating-is deposited, in any suitablemanner, upon thefsides vof the projections which are thensolderedintotheir metal holders and the individualelementsseparatedbybreaking the connecting web.

The above-'mentioned mould has the advantages that it is` easily splitin order to release the castings,\that=it needs no special alignmentwhen re-assembling andalso that-it ensures a constant size of castingWithout thenecessity of measuring accurate quantities of the material.

Having thus'descrbedfmy invention, what -1 claim is:

1. For use in a process for the manufacture of dry rectiers, a mouldcomprising, in combination, a block having a tapered bore extendingtherethrough; -a plug correspondingly tapered Afor ttin'gsaidPbore and.havingrecesses formed in a cutaway portion of the tapered surfacethereof; and a Crucible formed by one end of said plug -and the wall ofsaid bore extending beyond said endfa passage extending between saidCrucible Vand said cutaway portion.

2. For use in a process for the manufacture of dry'rectifiers, 'a'mouldcomprising, in combinan tion, a'blockfhaving a tapered circular boreextending therethrough; a cylindrical plug correspondingly tapered fortting into said bore and having at least one longitudinally extendingflat formed 'onthetapered surface thereof with a number of radialrecesses formed therein; and a cr-u'ciblefformed by one end of said plugand the Wall of said bore extending beyond said end, a passage,eXtending-'betvveensaid crucible'and a `chamber formed-between said`nat and `the fbore.

ALEXANDER JENKNS.

References Cited in-the file of this patent 'UNITED ST'TIiES zPA'ECENTS

